PC problem: why does it turn off by itself?

Question

Iron

First PC specs:
CPU - Intel Core 2 QUAD Q6600 (2.4GHz)
Video Card - Radeon HD4850 ​​(512MB)
RAM - 3GB DDR2
PSU - 500W
So, the computer is about eight years old, all this time it worked flawlessly. Three months ago, it started turning off by itself. Then one day it turned off and never turned back on. I took the PSU to the service, put it back - the computer works, everything is fine. A month later, it started to turn off again, but now only during games. At first it turned off when starting quite demanding games (BF3, Arma 3, Hitman: Absolution), now it turns off when starting any (Hitman Blood Money, GTA IV, Skyrim and others). Some games can be played for about half an hour (RimWorld), and then the computer still turns off. To turn it on again, you have to turn off the power supply, press the power button several times, then turn on the PSU and poke the button again until it turns on.
Oh, I forgot to add: it does not turn off while playing GTA V, but it crashes under heavy loads (character change, etc.).
Considering that the PC has been working without problems all this time, the version with a lack of PSU power can be thrown away, right? Then what is the problem and how to solve it?

Answer 1

The power supply is dying. Check with a known good one. According to the symptoms, the most likely candidate.
Much less likely candidates: the
motherboard capacitors have grown old and do not provide the necessary parameters
for overheating; the algorithm with turning off the power supply is uncharacteristic

Answer 2

Overheating of the processor or video card.
It was like when the fans were heavily covered with dust.
And thermal paste can lose heat over time.

Answer 3

Chipset overheating. How long ago was the paste changed? Cooling is passive?

Answer 4

In general, it would be worth running the RAM with a memtest, and monitoring the temperature at least with the same Everest (under different loads), in order to exclude overheating, etc.
Well, or yes, the conders flew.

Answer 5

If the motherboard, processor and video card with memory are 8 years old, give it to your parents to watch movies in the country. and buy yourself a simple new one, or a laptop.